1. Field of the Invention
The present invention relates to heat fusing apparatus for fusing toner images in an electrostatographic reproduction machine, such as a copier or printer. More particularly, the present invention relates to such a fusing apparatus that has heating and cooling means, thus making it particularly suitable for duplex or two-sided image fusing.
2. Description Relative to the Prior Art
In electrostatographic reproduction machines such as copiers and printers, it is well known to use a heat fusing apparatus to fuse toner images onto a side of a suitable substrate. The quality of such a fused image depends significantly on careful control of the fusing temperature of, and the amount of heat provided by, such a fusing apparatus. Examples of problems which occur when such careful control fails, are discussed for example in commonly assigned U.S. Pat. No. 4,963,943, issued Oct. 16, 1990 to Tamary. One such problem is "droop" that is, the delayed response to heat of a heated fuser roller, for which the '943 patent discloses a solution that includes carefully controlling the temperature of the fusing apparatus by simulating heat loss in the fuser roller with cool air.
Unfortunately, however, solving the droop problem, as such, is not enough to prevent other fusing problems for example in copiers and printers which produce and fuse double-pass duplex images on a substrate. Double pass duplex images are images on both sides of the substrate which are produced and fused one side at a time. Such other fusing problems for example include undesirable gloss which can result from reheating one of the duplex images during the fusing of the other image. Such undesirable gloss can be mottled gloss or any gloss that appears on the one side of the substrate but not on the other side thereof. Such an undesirable gloss problem will occur even in copiers and printers in which no "droop" problem exists.
More particularly, such a gloss problem is likely to occur in a copier or printer that uses a fusing apparatus which has an externally heated, axially unsupported fuser roller as disclosed for example in commonly assigned U.S. Pat. No. 4,905,050 issued Feb. 27, 1990 to Derimiggio et al. This is so because the pressure roller thereof which is used to contact and constrain the heated fuser roller itself becomes heated. Undesirable gloss results ordinarily when such a heated pressure contacts an already fused toner image on that side of a substrate contacting such a heated pressure roller.